1. Field of the Invention
The present invention relates to water jet propulsion modules and more particularly relates to a short inline annular module that is concentric with the axis of an impeller which directs water between parallel inlet and outlet openings through an annular duct which gradually reduces in size to discharge the water at the velocity that provides the maximum propulsive efficiency by maximizing the product of "Ideal Efficiency" and "Pumping Efficiency".
2. Description of the Prior Art
Assignee's Samuel U.S. Pat. No. 3,420,204 discloses a water jet reactive propulsion system which is designed to propel tracked amphibious military vehicles through water.
Rodler, Jr. U.S. Pat. Nos. 3,809,005 and 4,073,257 discloses two versions of jet propulsion systems of novel design wherein the water intake duct and the water discharge duct are connected by passageways which require approximately two 180.degree. reversals of direction of the propulsion water and provide a propulsion efficiency of about 20% higher than conventional water jets.
Conventional water jets are illustrated in Kenefich 3,174,454 and Trapp 3,306,046 wherein the water enters the water jet through a substantially horizontal opening and is discharged through a substantially vertical opening. The major disadvantage of the conventional water jets is their relatively low propulsive efficiencies. The power input to water jets is distributed to the following areas:
1. Inlet drag PA1 2. Internal flow losses PA1 3. Kinetic energy lost in the jet discharge PA1 4. Useful power output
The inlet drag is related to the square of the hull speed of a watercraft and is relatively negligible at speeds below 20 miles per hour but is a major factor in performance at high speeds.
Internal flow losses, such as inlet duct losses, impeller losses, stator losses, nozzle losses and steering losses in general relate to the square of the flow. Flow is primarily a function of power input and nozzle size. For a given application, increasing flow and decreasing pressure by use of a large nozzle will increase losses in this area. Said internal flow losses decrease the critical Pumping Efficiency. "Pumping Efficiency" is found by: ##EQU1##
Kinetic energy losses occur because the water starts in a static condition, but it is discharged from the water jet nozzle at a high velocity. The water jet thrust results from the reaction forces when this water is accelerated. The substantial kinetic energy in this high velocity water is an irrecoverable loss. "Ideal Efficiency" is used to qualify losses in this area. It is found as follows: EQU Ideal Efficiency=2.times.Hull Speed/(Hull speed+Jet speed.)
At any hull speed this equation shows that Ideal Efficiency can be improved only by decreasing Jet Speed. Decreasing jet speed requires greater flow to maintain a specific amount of thrust. Since the internal flow losses increase with increased flow, a careful trade off is required to optimize "Propulsive Efficiency" by maximizing the product of "Pumping Efficiency" and "Ideal Efficiency" while minimizing inlet drag by improved hydraulic design and minimized inlet area.